1. Field of the Invention
This invention relates generally to electronic article surveillance systems, and more particularly to electronic article surveillance systems that utilize a resonant tag that is affixed to an article being protected. In particular, the present invention relates to a system that utilizes a transmitter and an associated receiver at a gate or exit from an area being protected. The carrier frequency of the oscillator is swept through a range of frequencies including the resonant frequency of the tag, thereby causing the big to resonate periodically as the carrier frequency passes through the resonant frequency of the tag. The receiver picks up perturbations in the transmitted signals, also known as tag signals, caused by the presence of a tag in the transmitted field. The detected signals are processed to reduce noise and determine whether a valid tag signal is present. If a valid tag signal is received, an alarm is sounded.
2. Prior Art
Article surveillance systems are known, a typical swept frequency oscillator being shown in U.S. Pat. No. 3,868,669 to Minasy. Unfortunately, such systems have a tendency to respond to spurious signals that are present in the environment, particularly in an urban environment, and thus generate false alarms. Various attempts have been made to reduce such nuisance alarms. For example, Minasy periodically deactivates the system to discriminate against off-frequency signals, but this technique does not eliminate interfering signals at the tag frequency. Because of the small amplitude of a tag signal relative to the noise and other interfering signals generally present, it is difficult to make the system sensitive to tag signals without making it susceptible to interference from other signals. Various types of discrimination, such as frequency, amplitude, width, polarity, etc., have been used to discriminate the tag signal from noise and other signals, but these attempts have not been entirely successful.